The present invention relates to a light emitting device having a double-hetero structure including a first conductive type layer, an active layer, and a second conductive type layer stacked to each other, and to a method of fabricating the semiconductor light emitting device.
Various types of semiconductor light emitting devices are known. One of these types is configured by forming a low-temperature buffer layer overall on a sapphire substrate, forming an n-side contact layer made from Si-doped GaN on the low-temperature buffer layer, and stacking, on the n-side contact layer, an n-side cladding layer made from Si-doped GaN, an active layer made from Si-doped InGaN, a p-side cladding layer made from Mg-doped AlGaN, and a p-side contact layer made from Mg-doped GaN. Semiconductor light emitting devices having such a configuration have been commercially available in large quantities as blue or green LEDs (Light Emitting Diodes) for emission of light of blue or green, specifically, having wavelengths ranging from 450 nm to 530 nm.
In the case of forming a gallium nitride layer for fabricating a semiconductor light emitting device, crystal of gallium nitride is often grown on a sapphire substrate. In particular, a sapphire substrate with the C-plane taken as the principal plane of the substrate is used for growing crystal of gallium nitride thereon. In this case, the plane of the gallium nitride layer formed on the principal plane of the sapphire substrate also has the C-plane, and an active layer and cladding layers sandwiching the active layer, which are formed on planes parallel to the principal plane of the substrate, necessarily extend in planes parallel to the C-plane. The semiconductor light emitting device having such a structure that respective crystal layers are stacked in parallel to the principal plane of the substrate can obtain flatness required to form electrodes by making use of the flatness of the principal plane of the substrate.
The above-described technique for growing crystal of gallium nitride on a sapphire substrate, however, has a disadvantage that dislocations may densely exist in the crystal of gallium nitride due to lattice mismatch between the sapphire substrate and the crystal of gallium nitride grown thereon.
To solve such a disadvantage, an attempt has been made to form a low-temperature buffer layer on a substrate and grow crystal of gallium nitride thereon for suppressing crystal defects of the crystal of gallium nitride.
Japanese Patent Laid-open No. Hei 10-312971 discloses a technique of combining the formation of a low-temperature-buffer layer with epitaxial lateral overgrowth (ELO) for reducing crystal defects.
Japanese Patent Laid-open No. Hei 10-321910 discloses a semiconductor light emitting device configured by forming a hexagonal prismatic structure having a side plane composed of the (10-10) plane or the (1-100) plane [M-plane] perpendicular to the principal plane of a substrate, and forming a light emission region on the hexagonal prismatic structure in such a manner that the region extends in the direction perpendicular to the principal plane of the substrate. With this configuration, since an active layer and the like forming the light emission region extend in the direction perpendicular to the principal plane of the substrate, it is possible to suppress crystal defects and dislocations due to lattice mismatch with the substrate, and to reduce strain due to difference in thermal expansion coefficient therebetween.
The technique forming the hexagonal prismatic structure extending in the direction perpendicular to the principal plane of a substrate disclosed in Japanese Patent Laid-open No. Hei 10-321910, however, has the following problem. In this technique, after being formed by a hydride vapor phase epitaxy (HVPE) process, a layer structure is subjected to dry etching for obtaining the side plane composed of the (10-10) plane or the (1-101) plane [M-plane]. In general, such dry etching necessarily damages the crystal plane. Accordingly, this technique has a disadvantage that characteristics of the crystal is degraded by dry etching, although it has an advantage in suppressing threading dislocations extending from the substrate side. Another disadvantage is that since dry etching is additionally performed, the number of steps is increased.
In the case of forming a crystal layer on the C+-plane of a sapphire substrate by selective growth, the crystal growth has a tapered shape surrounded by the (1-101) plane, that is, the S-plane (for example, see Paragraph 0009 in Specification of Japanese Patent No. 2830814). This technique, however, has a disadvantage that a flat plane required to form electrodes is not obtained. Such a structure, therefore, has not been positively used for electronic devices or light emitting devices, and used only as an underlying layer for selective growth of a crystal structure.
To fabricate devices of a type including layers extending within planes parallel to the principal plane of a substrate, it is important to form each flat plane for keeping desirable crystallinity, and therefore, the device structure tends to have electrodes and the like extending in the horizontal direction. Accordingly, in the case of separating the devices from each other, the device structure must be cut into chips by a dicer, with a result that it takes a lot of time to cut the device structure into chips and further it is very difficult to cut the device structure into chips while avoiding electrodes and the like spread in the horizontal direction. Also, since a sapphire substrate and a nitride such as GaN are too hard to cut, dicing of the device structure, that is, the sapphire substrate and the nitride requires a cutting allowance of at least about 20 μm. This makes it even more difficult to cut the device structure into the micro-sized chips.
Light emitting devices of a type, which is configured by forming an active layer made from a gallium nitride based material on a plane parallel to the principal plane of a substrate with the C+-plane taken as the principal plane of the substrate, has the following problem. Since Ga has only one bond to nitrogen atoms on the C+-plane, nitrogen atoms are liable to dissociate from the crystal plane parallel to the C+-plane, thereby failing to increase an effective V/III ratio. As a result, the crystallinity for forming the light emitting device is insufficient to improve the performance thereof.